i915: Add a Kconfig option to turn on i915.preliminary_hw_support by default

When building kernels for a preliminary hardware target, having to add a
kernel command-line option can prove inconvenient.  Add a Kconfig option
that changes the default of this option to 1.
Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Pimp the Kconfig help text a bit as suggested by Damien in
his 2nd review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/Kconfig

@@ -168,6 +168,17 @@ config DRM_I915_KMS
 	  the driver to bind to PCI devices, which precludes loading things
 	  like intelfb.
 
+config DRM_I915_PRELIMINARY_HW_SUPPORT
+	bool "Enable preliminary support for prerelease Intel hardware by default"
+	depends on DRM_I915
+	help
+	  Choose this option if you have prerelease Intel hardware and want the
+	  i915 driver to support it by default.  You can enable such support at
+	  runtime with the module option i915.preliminary_hw_support=1; this
+	  option changes the default for that module option.
+
+	  If in doubt, say "N".
+
 config DRM_MGA
 	tristate "Matrox g200/g400"
 	depends on DRM && PCI

drivers/gpu/drm/i915/i915_drv.c

@@ -122,10 +122,10 @@ int i915_enable_psr __read_mostly = 0;
 module_param_named(enable_psr, i915_enable_psr, int, 0600);
 MODULE_PARM_DESC(enable_psr, "Enable PSR (default: false)");
 
-unsigned int i915_preliminary_hw_support __read_mostly = 0;
+unsigned int i915_preliminary_hw_support __read_mostly = IS_ENABLED(CONFIG_DRM_I915_PRELIMINARY_HW_SUPPORT);
 module_param_named(preliminary_hw_support, i915_preliminary_hw_support, int, 0600);
 MODULE_PARM_DESC(preliminary_hw_support,
-		"Enable preliminary hardware support. (default: false)");
+		"Enable preliminary hardware support.");
 
 int i915_disable_power_well __read_mostly = 1;
 module_param_named(disable_power_well, i915_disable_power_well, int, 0600);